Any+Time Date/Time Picker - This javascript widget built on the Any+Time Library has more customizations than any date picker we've ever seen. In addition to fully customizable date/time fields, you can choose which fields to display, place limits on dates, start your week at any day and even accept dates in BC if you ever have the need. There's also a variety of styling options from the standard css expectations to theming options through jquery Theming, Theme Switcher or Themeroller. Any+Time also offers an older version for the Prototype library.